1. Field of the Inventions
The present inventions relate to multiple-cylinder engines for outboard motors, and more particularly, the exhaust systems of such engines.
2. Description of the Related Art
A conventional multiple-cylinder engine for an outboard motor is disclosed in Japanese Patent Document JP-A-Hei 9-49425. According to this Patent Document, an engine body of the engine includes a crankcase for supporting a crankshaft which has a vertical axis and plural cylinders that are vertically arranged, each cylinder horizontally extending outwardly from the crankcase. An exhaust port is formed for each cylinder in the cylinder head. Plural exhaust passages are formed, each of which extend from a downstream end of the respective exhaust port.
The downstream ends of the exhaust passages extending from the respective cylinders are joined to each other to form plural, joined-flow exhaust passages. The ignition order of these cylinders is not sequential. The downstream ends of these exhaust passages are joined to each other and then pass through expansion chambers and then to the atmosphere.
While the engine is operating, exhaust gas from each cylinder passes through the exhaust passages and flows into the joined flow exhaust passage. In this case, as previously noted, the ignition order of the cylinders which feed exhaust gas into the exhaust passages in communication with one joined flow exhaust passage, is not sequential. Thus, the timings at which exhaust gas is emitted from the cylinders differ from each other. This prevents exhaust gas from the cylinders from interfering with each other in the joined flow exhaust passage, resulting in excellent exhaust efficiency.
After that, the exhaust gas from the cylinders is joined to each other at the downstream ends of the joined flow exhaust passages, then passes through the expansion chamber to be muffled, and is discharged to the atmosphere.